The invention relates generally to application of coatings to fluorescent lamps, and more specifically to application of protective coatings for containment of shattered glass and interior components of the lamps in the event of breakage.
It is known in the art that application of a protective coating to a fluorescent lamp can enable containment of shattered glass in the event of breakage. Fluorescent lamps with protective coatings are widely used in locations where breakage may contaminate food products, and in other environments where particular precautions against environmental contamination are needed.
In the past, a variety of methods have been employed to apply coatings of various kinds to fluorescent lamps, as well as to incandescent lamps and photoflash lamps.
U.S. Pat. No. 3,959,525 discusses dip-coating of photoflash lamps to apply a protective lacquer coating to reinforce a glass envelope and improve its containment capability. This patent describes employment of dip-coating to apply cellulose acetate or polycarbonate coatings, and states that fluidized bed coating techniques have been employed to apply cryogenically ground polycarbonate resin and other thermoplastic materials.
U.S. Pat. No. 3,902,946 discloses spraying of a silicone release agent onto the glass envelope of a photoflash lamp, then placing the lamp in a preformed sleeve of thermoplastic material which is subsequently vacuum-formed onto the glass envelope.
U.S. Pat. No. 3,018,187 describes application of silicone coatings to the envelopes of fluorescent lamps by means of dip or spray systems. The function of the silicone coating is to prevent moisture from condensing on or otherwise contacting the envelope in order to minimize problems encountered in starting "rapid-start" and "instant-start" fluorescent lamps under high humidity conditions.
U.S. Pat. No. 3,102,049 also discusses application of a water repellent film to a "rapid start" fluorescent lamp, to avoid start-up problems cause by high atmospheric humidity. This patent discloses a coating method in which fluorescent lamps are rolled through the solution to cover the entire external surface of the glass envelope, with guide bars placed in the solution at a depth of about 1/4 inch below the liquid so that the ends of the lamp and those parts of the lamp which will be employed in making electrical contacts will not be coated.
It is believed that silicone solutions have also been applied to tubular fluorescent lamps by rollers, with lower portions of the rollers rotating through a silicone solution or emulsion, and upper portions of the rollers in rolling contact with rotating lamps rotated about their axes.
U.S. Pat. Nos. 4,507,332, 4,506,189, and 4,499,850 discuss coating of fluorescent lamps with Surlyn powder in a fluidized bed. The stated purpose of the coating is to collect glass shards upon the glass envelope being broken and maintaining the glass shards and end caps in association within the coating.
A polyvinylchloride (PVC) coating can be applied to fluorescent lamps by a dip-coating process, and such a coating is capable of providing highly effective containment of shattered glass and internal lamp components in the event of breakage. It has been found that dip-coating of fluorescent lamps in a PVC plastisol provides an effective means of applying coating material to the lamps. However, in the past, commercial methods for coating of fluorescent lamps with PVC plastisol have been relatively labor-intensive. Known prior art methods of applying PVC plastisol to fluorescent lamps have been batch-coating operations, wherein a batch of fluorescent lamps are supported for rotation on a movable rack, and the rack is moved manually to effect the necessary steps for coating. Such methods have inherent inefficiencies due to the time and care required load and unload the relatively fragile lamps, and to perform other steps on the lamps in each batch individually.
In a known batch-coating method, the rack has pairs of adapters to support the lamps at their opposite ends by engagement with the end caps and electrical connecting pins, and one of each pair of adapters is driven to rotate the lamps about their respective axes. A primer is manually applied about the periphery of the end caps to improve bonding of the coating material to the end caps. The primer is applied by placing a hand-held brush in contact with each end cap as the lamps rotate. The rack and lamps are inserted in an oven for preheating for a brief period of time, then withdrawn from the oven by returning the rack to its original position. The rack is then pivoted to dip the lamps into contact with the surface of a bath of liquid plastisol while they are rotated about their axes. The electrical contact pins are maintained above the level of the plastisol bath as the lamps are rotated. After the tubular glass central portion of the lamp and peripheral regions of the end caps have been coated with the liquid plastisol, the lamps are lifted from the bath and replaced in the oven, while still rotating, to effect gelation of the liquid plastisol into a tough, transparent solid coating. The rack is then manually withdrawn from the oven once again, and after the lamps have cooled by exposure to ambient air for a short period of time, adhesive-backed labels are manually applied to the lamps. The lamps are then manually removed from the adapters.
One of the problems in coating with PVC plastisol as described above is that the operator must periodically replenish the bath to maintain the surface at a satisfactory level. Monitoring of the level is complicated by the fact that the level increases somewhat due to displacement as the lamps are lowered into the bath, and due to the relatively high viscosity of the plastisol. If the level is too high relative to the lamps, the result may be an unacceptable buildup of plastisol on the electrical connecting pins, end caps, and/or the adapters. The high viscosity of the plastisol exacerbates this problem. After curing, such buildups may be difficult to remove, and may require individual manual removal with a knife of other appropriate tool.
It is a general object of the invention to provide a more efficient and economical method and apparatus for applying a protective coating to fluorescent lamps to contain the glass envelopes and internal lamp components in the event of failure of the glass envelope. Among the problems addressed by the invention in this connection are control of the level of the liquid plastisol in the bath, and take-off of coated lamps in a more efficient manner without increased breakage.
Another object of the invention concerns labeling of the coated fluorescent lamps. It has been found that labels which have been applied conventionally to such lamps are subject to removal and misuse after the end of the useful life of the lamp. Accordingly, it is a further object of the invention to provide a tamper-resistant label for coated fluorescent lamps.